Hepatitis B virus (HBV) infection is a major, worldwide health problem. It is estimated that 300,000 individuals in the U.S., alone, are infected each year. Approximately 10% of infected individuals are unable to clear the virus infection, often resulting in residual chronic infection and its devastating consequences, cirrhosis and primary liver cancer. Worldwide, over 300 million people are chronically infected with HBV. Interferon therapy has resulted in the clearance of infection in 10-30% of chronic HBV patients, but the majority remain refractory to treatment. Although effective vaccines are available, they have only recently begun to be distributed widely in the U.S., mostly to children. HBV, like other viruses, is most vulnerable to intervention before it enters its major target cell, the hepatocyte. This project is aimed at identifying and understanding these early interactions of HBV, especially its attachment to hepatocytes. No cell lines are capable of supporting HBV replication due to an early block, probably the absence of the HBV receptor. If the HBV receptor could be identified, agents that block virus attachment to this receptor might be designed for therapeutic use. Furthermore, identification of the HBV receptor might lead to cell culture and transgenic mouse systems in which to study the early stages of HBV infection to develop new antiviral drugs. A number of HBV receptor "candidates" have been described, but none has been demonstrated to be physiologically relevant. In this proposal, the early events in HBV infection will be studied in a primary human hepatocyte system. These are the only cultured cells that can be infected with HBV. Preliminary experiments have indicated that a prolonged inoculation time is necessary for maximal infection. The first two specific aims of this proposal are to determine the cause of this slow rate of attachment, to test the candidate HBV receptors proposed by this laboratory and by others, and to use a monoclonal antibody blocking approach to identify the HBV receptor. The third specific aim is to identify the HBV receptor by genetic means. A cells expressing a cDNA library prepared from human liver tissue will be inoculated with HBV. Infected clones will isolated and the receptor cDNA isolated.